1. Field of the Invention
The present invention relates to a feed forward amplifier used for a base station for mobile communication equipment.
2. Description of the Related Art
In recent years, power amplifiers providing higher outputs and better linearity have been required for base stations for mobile communication equipment. Despite this demand for high outputs, no power amplifiers have been developed yet that enable a single device to output several hundred W, so a plurality of power amplifiers are combined in parallel to meet this demand. The linearity has been realized by providing distortion compensation based on the feed forward method.
FIG. 6 shows an example of a configuration of a conventional feed forward amplifier. In this figure, 1 is an input terminal, 2 is an output terminal, 3 and 6 are power dividers, 4 is a distortion canceling power combiner, 5 and 14 are vector adjusters, 7 is an output power combiner, 8 and 9 are power amplifiers, 11 and 13 are delay circuits, 12 is a distortion detecting power combiner, and 15 is an auxiliary amplifier. The power divider 6, the output power combiner 7, and the power amplifiers 8 and 9 constitute a main amplifier 10. In addition, references (a) to (j) added to the power divider 3, the distortion canceling power combiner 4, and the distortion detecting power combiner 12 designate ports.
The operation of the feed forward amplifier of this configuration is described below.
First, the power divider 3 divides into two an input signal (see FIG. 7(a)) input from the input terminal 1 and containing multi-channel components. One of the divided output signals is transferred via the vector adjuster 5 to the main amplifier 10, which then amplifies this signal. The amplified signal is input to the port (d) of the distortion detecting power combiner 12 (see FIG. 7(b)). Due to non-linearity of the main amplifier 10, the input signal (see FIG. 7(b)) contains not only the input signal components but also distortion components resulting from inter-modulation. In addition, the other divided signal is input to the port (e) of the distortion detecting power combiner 12 through the delay circuit 11. By adjusting the vector adjuster 5 and the delay circuit 11 so that the input signal components of the signals input to the ports (d) and (e) have an equal amplitude and opposite phases, a signal consisting of only the distortion components due to the canceled input signal components (see FIG. 7(c)) is output from the port (g).
Next, a signal output from the port (f) and containing both the input signal components and the distortion components is input to the port (h) of the distortion canceling power combiner 4 via the delay circuit 13. In addition, the signal with the distortion components output from the port (g) is transferred via the vector adjuster 14 to the auxiliary amplifier 15, which then amplifies this signal. The amplified signal is then input to the port (i) of the distortion canceling power combiner 4. By adjusting the vector adjuster 14 and the delay circuit 13 so that the distortion components of the signals input to the ports (h) and (i) have an equal amplitude and opposite phases, a signal consisting of only the input signal components due to the canceled distortion components (see FIG. 7(d)) is output from the port (j) to the output terminal 2.
FIGS. 7(a) to 7(d) show the frequency spectra of the signals at the ports (a), (d), (g), and (j) of FIG. 6.
In this configuration, however, the output power of the main amplifier 10 decreases due to losses in the distortion detecting power combiner 12 and delay circuit 13 to reduce the efficiency of the entire feed forward amplifier.